Various protein texturisation processes have been used for some time in the manufacture of various food products, such as in the manufacture of sausages, kamaboko, meat analogs and seafood analogs. A fibrous texture may be obtained by various means, including extrusion cooking at low moisture levels (typically 10-30% by weight). Extrusion cooking at high moisture levels (e.g. typically 30 to 80% water by weight) is a relatively new technique, which is finding use mainly in the field of texturization of protein food products.
High moisture extrusion cooking has been discussed as a means of restructuring various natural protein sources, such as fish mince, surimi, de-boned meats, soy flours, concentrates, cereal flours, dairy proteins and the like, in order to obtain cohesive fibrous or lamellar structures (e.g. see “New Protein Texturization Processes by Extrusion Cooking at High Moisture Levels” by J C Cheftel et al, Food Reviews International, 8 (2), 235-275 (1992) published by Marcel Dekker, Inc.).
Unlike low moisture extrusion cooking, high moisture extrusion cooking requires the use of cooling dies for cooling, gelling and/or solidifying the food extrudate issuing from the food extruder. A cooling die dissipates the thermal and mechanical energy accumulated in the food mix, increases the viscosity of the mix, and prevents extrudate steam flash at the die outlet which would cause unwanted expansion of said extrudate.
Three main types of cooling dies are known for use in this field of technology/application. Most commonly known are elongated rectangular or cylindrical cooling dies, with a rectangular or cylindrical duct extending along the length of the die. The regions surrounding the duct are cooled with water thereby enabling the extruded food product passing through to be cooled. There are also annular cooling dies in which the internal cavity has an annular cross-section defined by an inner core and an outer cylinder. The inner core and outer cylinder are cooled, thereby enabling the food product passing through the cavity to be cooled.
There are also known multi-channel (or duct) cooling dies where the extrudate exiting the extruder is ‘split’ and made to flow into a number of individual cooling ducts disposed about a longitudinal axis of the die such that individual strands of solidified, textured product are delivered at the cooling die outlet.
Thus, multi-channel cooling dies have the potential for increased throughput without adversely affecting the product's characteristics and increasing the cooling time, which would be the result of using a single duct die with an increased cross-sectional area.
Attempts have also been made to increase the capacity of cooling dies through the use of higher flow rates with single-duct cooling dies of greater cross-sectional areas. This measure necessitates longer cooling dies. This has a number of adverse consequences. For instance, longer cooling dies increase the likelihood of inconsistencies arising in the food product and structure blockages occurring in the cooling die. Also, such dies obviously take up more area or floor space of the production plant, which in turn increases costs.
Japanese patent application No. 4-214049 (publication no. 6-62821) discloses a multi-channel cooling die which is used in the extrusion of thin, thread-like food products from high moisture content proteinaceous raw materials. The cooling die is essentially constructed like a typical shell-and-tube heat exchanger, wherein the shell covers at the axial ends of the cylindrical shell are replaced with purpose built end plates. The inlet end plate is flanged to the extruder's die plate holder, while the other end plate is similar in layout to the stationary tube sheet of the heat exchanger, i.e. a multiple-orifice plate in which the ends of the plurality of inner tubes are wedged and supported.
The plurality of thin-walled inner tubes employed in such type of cooling die ensures efficient cooling at higher throughput rates of extrudate. It is said that the individual tubes possess high pressure resistance thereby enabling processing of greater amounts of raw materials as compared with conventional, single cavity cooling dies.
One shortcoming of this type of cooling die is the need to use implements such as long rods for cleaning the individual inner tubes through which the extrudate flows during processing. The smooth surface of the tubes can be damaged during the cleaning process (due to their length), which may result in irregular loading of individual tubes from the extruder as a consequence of increased surface roughness (and back pressure) at individual tubes. Also, where one of the tubes is damaged to an extent that it no longer provides a flow path for the molten extrudate, it may be necessary to replace the entire cooling die or individual tubes. The replacement process is potentially time-consuming and labour intensive, as the individual tubes are received in airtight manner at the end plates of the cylindrical shell. Therefore, all tubes have to be removed and refitted in order to exchange any one of them.
Patent Application PCT/AU01/00011, in the name of Effem Foods Pty Ltd discloses a cooling die for use with a food extruder, which enables greater manufacturing output without substantially increasing the cross-sectional area or length of the extrudate flow cavity, when compared to single cavity cooling dies used in the art, by providing a multi-channel cooling die which addresses some or all of the disadvantages perceived to exist with shell-and-tube type cooling dies. The cooling die body consists of a number of stacked plate elements, which are braced together. Each plate has through-holes, which align with corresponding through-holes of adjacent plates thereby to define multiple product flow channels (ducts) and coolant flow channels. However there are some disadvantages associated with this particular design, outlined as follows:
Coolant Flow
In the stacked plate cooling die assembly, coolant flows through a number of ducts that are formed when the required number of die-core elements are bolted together. The diameter of these channels varies according to the radial position thereof from the longitudinal axis of the die. The design is chosen such that the distance between the coolant channels and the product channels remains constant over the entire cross-section of the die. However, it also means that the outer cooling channels will have the higher coolant flow rate. Unless additional coolant flow restriction elements are employed to achieve constant and equivalent flow through all cooling medium channels, such design will have adverse consequences for product quality and consistency, product exiting the cooling die would tend to be heterogeneous with the portion of product closer to the center of the cooling die being slightly more expanded (due to poorer cooling) than the product further from the center. Product also will occasionally twist due to a velocity gradient from inner to outer radius of the product channels. In addition to this, uneven heat transfer can result in some product channels running more slowly than others. This results in product passing through the cooling die at various residence times. Some product may therefore be over expanded while other product is not sufficiently expanded. An example of operational difficulties that may arise due to this effect is that the flow in some channels may stop completely while the flow in the others increases to an extent where it does not receive adequate cooling, adversely affecting product quality.
Product Flow From Extruder to Die
Product flow from extruder to die plays a very important role in cooling die stability. It is important for the average flow path from extruder outlet to cooling die product inlet to have as small a standard deviation as possible. Alternatively, the difference between the shortest path and longest path from extruder to cooling die channel inlet should be as small as possible. Large differences have been found to exacerbate the creation of dead spaces in the transition piece between the extruder outlet and cooling die inlet. This in turn leads to the development of burnt product that can lead to downgraded product or blocked primary die holes.
Die Assembly & Cleaning
As noted above, the cooling die disclosed in PCT/AU01/00011 consists of a number of plates; stacked together to form the cooling die body, in which are formed a plurality of product flow channels and adjacent coolant channels. In a typical configuration, 20 plates are used. A disadvantage associated with this form of construction is that assembly of this die time-consuming, and requires significant operator skill to position all the plates correctly and tension the clamping bolts. Dismantling of the cooling die for cleaning purposes may also take significant time, particularly if the product remaining in the cooling die at the conclusion of a run solidifies in the channels, effectively bonding the plates together. Such solidified product may potentially be difficult and time-consuming to remove. Consequently, turnaround times for this type of cooling die may be up to about 4 hours. There are significant advantages associated with reducing this turn-around time.
Running Cost
The cooling die design disclosed in PCT/AU01/00011 relies on direct contact between adjacent plates to seal both the extrudate and coolant channels, unless additional seal elements are used at each channel joint or interface. Slight wear on the surfaces of the plates could result in extrudate leaking into coolant channels, or to the outside, or coolant leaking into extrudate channels, or to the outside. Even small levels of leakage create quality or stability problems and so damaged plates are replaced regularly.